March 24, 2016
JAMA : The Journal of the American Medical Association
TAKE-HOME MESSAGE
- The authors of this review evaluated the epidemiology of recent outbreaks of measles and pertussis to assess the effect of vaccine refusal or exemption on disease rates. No measles vaccine had been given to 56.8% of 1416 individuals diagnosed with measles and, in patients for whom detailed vaccine data were available, 70.6% of exemptions were for nonmedical reasons rather than medical contraindications. The proportion of unvaccinated or under-vaccinated individuals in 32 pertussis outbreaks ranged from 24% to 45%, although outbreaks were also seen in highly vaccinated populations, suggesting waning of acquired immunity. In cases where vaccination data were available, reports were that 59% to 93% of unvaccinated individuals had declined vaccination
- Vaccine refusal has led to an increased risk of measles in both vaccinated and unvaccinated people, following prior elimination of the disease. While the resurgence in pertussis is partly due to waning immunity, vaccine refusal does increase the risk of infection.
Primary Care
Written by Jonathan Temte MD, MS, PhD
When I think of the role of vaccines in the prevention and control of communicable diseases, I keep in mind the formula for exponential growth. The real driver here is the basic reproduction number (Ro)—a descriptor of the number of cases generated by the introduction of index cases in a non-immune population. For measles, Ro is spectacularly high at 12 to 18; pertussis is moderate at 5.5; Ebola is lower at 1.5 to 2.5. Vaccines, in effect, serve to lower Ro by making the population less susceptible.
Accordingly, within two generations of the introduction of measles vaccine in the United States, measles went from affecting nearly everyone to being eliminated; sustained transmission was prevented due to high vaccine coverage. A similar thing happened with pertussis through the mid-1990s. A move to use acellular pertussis vaccine, however, allowed some resurgence of pertussis.
In a nice review of measles and pertussis, Phadke and colleagues1 ask what happens when people—for nonmedical reasons—refuse vaccines. Two doses of measles vaccine (a component of MMR) provide high levels of long-lasting immunity. Multiple doses of acellular pertussis (DTaP, Tdap) provide good levels of protection, but for a limited duration. Using these two distinctly different vaccines, the authors provide much commonality.
Vaccine refusal for nonmedical reasons was noted in about 42% of measles cases, for which sufficient information was available, since 2000. More importantly, individuals with vaccine exemptions were 22 to 35 times more likely to contract measles. Similarly, the risk of pertussis was 5.9 to 20 times higher for those with exemptions.
Because communicable diseases exist within chains of transmission, people more likely to acquire infections are much more likely to transmit to the next person. As we consider the increasing phenomenon of vaccine refusal and hesitancy, we need to think back to the basic epidemiology. It isn’t only an individual choice, but a communal one. When counseling patients and parents, I routinely emphasize the safety and effectiveness of vaccines for individual protection. I then provide a reminder that we do not live in bubbles but interact with dozens of other people on a daily basis, and some of those individuals may not fare well with vaccine-preventable disease.
Reference
Phadke VK, Bednarczyk RA, Salmon DA, Omer SB. Association between vaccine refusal and vaccine-preventable diseases in the United States: a review of measles and pertussis. JAMA. 2016;315(11):1149-1158.
Abstract
IMPORTANCE
Parents hesitant to vaccinate their children may delay routine immunizations or seek exemptions from state vaccine mandates. Recent outbreaks of vaccine-preventable diseases in the United States have drawn attention to this phenomenon. Improved understanding of the association between vaccine refusal and the epidemiology of these diseases is needed.
OBJECTIVE
To review the published literature to evaluate the association between vaccine delay, refusal, or exemption and the epidemiology of measles and pertussis, 2 vaccine-preventable diseases with recent US outbreaks.
EVIDENCE REVIEW
Search of PubMed through November 30, 2015, for reports of US measles outbreaks that have occurred since measles was declared eliminated in the United States (after January 1, 2000), endemic and epidemic pertussis since the lowest point in US pertussis incidence (after January 1, 1977), and for studies that assessed disease risk in the context of vaccine delay or exemption.
FINDINGS
We identified 18 published measles studies (9 annual summaries and 9 outbreak reports), which described 1416 measles cases (individual age range, 2 weeks-84 years; 178 cases younger than 12 months) and more than half (56.8%) had no history of measles vaccination. Of the 970 measles cases with detailed vaccination data, 574 cases were unvaccinated despite being vaccine eligible and 405 (70.6%) of these had nonmedical exemptions (eg, exemptions for religious or philosophical reasons, as opposed to medical contraindications; 41.8% of total). Among 32 reports of pertussis outbreaks, which included 10 609 individuals for whom vaccination status was reported (age range, 10 days-87 years), the 5 largest statewide epidemics had substantial proportions (range, 24%-45%) of unvaccinated or undervaccinated individuals. However, several pertussis outbreaks also occurred in highly vaccinated populations, indicating waning immunity. Nine reports (describing 12 outbreaks) provided detailed vaccination data on unimmunized cases; among 8 of these outbreaks from 59% through 93% of unvaccinated individuals were intentionally unvaccinated.
CONCLUSIONS AND RELEVANCE
A substantial proportion of the US measles cases in the era after elimination were intentionally unvaccinated. The phenomenon of vaccine refusal was associated with an increased risk for measles among people who refuse vaccines and among fully vaccinated individuals. Although pertussis resurgence has been attributed to waning immunity and other factors, vaccine refusal was still associated with an increased risk for pertussis in some populations.